A disc cartridge is used to contain a disc-like shaped recording medium (hereinafter, referred to as a disc) such as optical discs or magnetic discs, for easy handling in mounting the disc in a recording/reproducing apparatus, and for dust control. The disc cartridge is provided with an opening so that in recording or reproducing data, the disc is accessible to a recording/reproducing head, such as a magnetic head, and a turn table for rotating the disc, of the recording/reproducing apparatus. For preventing dust and the like from coming inside of the cartridge through the opening to reach the disc, the opening is generally provided with a shutter capable of arbitrarily opening and closing.
Specifically, when the disc cartridge is loaded in the recording/reproducing apparatus, the disc cartridge exposes part of the disc via the opening, so as to allow reproduction of data stored on the disc, and recording of data onto the disc. On the other hand, when the disc cartridge is not loaded in the recording/reproducing apparatus, the shutter is closed to cover the opening, thereby protecting the disc from dust and the like.
For example, a disc cartridge containing a magneto-optical disc therein is known as this kind of disc cartridge having such a shutter. Recording and Reproduction of magneto-optical discs use a magnet head and an optical head of a recording/reproducing apparatus. Discussed below is a disc cartridge containing a magnet optical disc of 86 mm diameter, referring to FIGS. 8 to 12.
As shown in FIG. 9, a disc cartridge 61 (see FIGS. 8(a) and 8(b)) contains a magnet-optical disc (hereinafter, a disc) 62 in an interior space of a cartridge main body constituted by combining an upper half 70 and a lower half 80. The disc 62 is capable of rotating arbitrarily in the interior space of the cartridge main body. Further, in the cartridge main body, a shutter 90, which is capable of opening and closing arbitrarily as described above, is provided to openings 75 and 85, which are provided for writing and reading a data signal into/from the disc 62 (hereinafter, this operation is just referred to as recording and reproduction of data).
The shutter 90 is, as shown in FIG. 9, attached to a shutter slider 95, which slides along side sections of the upper and lower halves 70 and 80, and is capable of sliding on the cartridge main body by using a shutter holding plate 89 provided on the lower half 80. The shutter 90 moves in synchronism with movement of the shutter slider 95, so as to open or close the openings 75 and 85.
Moreover, as shown in FIGS. 8(a) and 8(b), a shutter spring 99 (see FIG. 9) is provided between an end of the shutter slider 95 and the cartridge main body, in order to realize movement of the shutter slider 95 along the side sections of the upper and lower halves 70 and 80. By utilizing extension and contraction of the shutter sprig 99, the shutter slider 95 slides along the side sections of the upper and lower halves 70 and 80, so as to slide the shutter 90 on the cartridge main body.
The opening 75 is, as shown in FIG. 9, provided on the upper half 70 of the disc cartridge 61 so that the magnetic head of the recording/reproducing apparatus can access to the disc 62. Meanwhile, the opening 85 is provided on the lower half 80 so that the optical head and the turn table can access to the disc 62. In addition, the disc cartridge 61 is provided with a light protector 63 for prevention of erroneous deletion of data stored in the disc 62 in reproducing the disc 62.
The disc cartridge 61 having the above arrangement performs opening and closing operation as follows.
In the disc cartridge 61, when the shutter spring 99 (see FIG. 9) is substantially straightly extended, the shutter 90 covers the openings 75 and 85, as shown in FIGS. 8(a) and 10, thereby closing the openings 75 and 85. On the other hand, when the openings 75 and 85 are opened so that part of the disc 62 is exposed via the openings 75 and 85, the shutter spring 99 is folded into a substantially V-like shape. This causes the shutter 90 to slide in synchronism with the shutter slider 95, so as to expose the disc 62 via the openings 75 and 85. Then, by using the magnetic head and the optical head, the recording and reproduction of data are carried out on that part of a signal recording surface of the disc 62 that is exposed through the openings 75 and 85.
Meanwhile, Japanese Publication of Unexamined Patent Application, Tokukai, No. 2000-30396 (published on Jan. 28, 2000), as shown in FIGS. 11(a) and 11(b), discloses a disc cartridge 101 including an opening 115 provided with two shutters 110a and 110b, which open and close by rotation. In the disc cartridge 101, as shown in FIG. 12, a lower half 103 of a cartridge main body is provided with a recess section 103a through which the two shutters 110a and 110b slide. Further, a lower half cover 104 is provided so that the shutters 110a and 110b are held in the recess section 103a. 
Furthermore, the shutter 110a, as shown in FIGS. 11(a) and 12, is provided with a canopy section 110x in a vicinity of a side surface against which the shutter 110b will abut. Thus, the canopy section 110x of the shutter 110a overlaps on the shutter 110b, when the shutters 110a and 110b close.
In the disc cartridge having the above arrangement, by sliding the shutters 110a and 110b through the recess section 103a, the shutters 110a and 110b are opened and closed by rotation. When the shutters 110a and 110b open, the shutters 110a and 110b move apart from each other as shown in FIG. 11(b), thereby exposing part of a disc 102 via the opening 115.
On the contrary, when the two shutters 110a and 110b close, the two shutters 110a and 110b abut against each other as shown in FIG. 11(a), thereby covering the opening 115. Here, as shown in FIG. 12, the canopy section 110x provided on the shutter 110a covers an area where the shutters 110a and 110b abut against each other. This arrangement prevents invasion of dust through a gap in the area where the shutters 110a and 110b abut against each other when the shutters 110a and 110b close.
As described above, the opening and closing of the shutter 90 of the disc cartridge 61, and those of the shutters 110a and 110b of the disc cartridge 90 are respectively performed by sliding the shutters along the cartridge main bodies.
For smooth opening and closing of the shutter 90, the disc cartridge 61 is provided with gaps β between the shutter 90 and the upper and lower halves 70 and 80, respectively, as shown in FIG. 10. If the shutter 90 and the upper and lower halves 70 and 80 were in full contact, a large friction would be caused therebetween. This friction would lead to difficulty in sliding the shutter 90 thereby failing to attain the smooth opening and closing. Moreover, the upper and lower halves 70 and 80 should have a satisfactorily flat surface in order to let the shutter 90 to slide thereon where the shutter 90 and the upper and lower halves 70 and 80 are in full contact. Therefore, in the disc cartridge 61, the smooth opening and closing the shutter 90 are realized by providing the quite small gaps β between the shutter 90 and the upper and lower halves 70 and 80, respectively.
Moreover, in the disc cartridge 101, as shown in FIG. 12, gaps γ are provided between the lower halve 103 and the shutters 110a and 110b, thereby enabling smooth opening and closing of the shutters 110a and 110b. 
However, in the conventional disc cartridge 61, which is, as shown in FIG. 10, provided with the quite small gaps β respectively between the shutter 90 and the upper and lower halves 70 and 80, there is a risk that dust and the like would invade into the cartridge 61 and reach the disc 62 via the gaps β and the openings 75 and 85. Furthermore, there is a risk that the sliding of the shutter 90 along the upper and lower halves 70 and 80 causes friction respectively at an edge section 75a of the opening 75 and art edge section 85a of the opening 85, thereby producing friction powder. There is a possibility that the friction powder could pass through the openings 75 and 85 so as to attach onto the surface of the disc 62. Dust, powder and the like attached onto the surface of the disc 62 may cause troubles in recording and reproducing, thereby leading to failure in recording and reproducing data.
On the other hand, the disc cartridge 101 disclosed in Tokukai No. 2000-30396 is provided with the shutters 110a and 110b (FIG. 12) between the recess section 103a provided in the lower half 103 and the lower half cover 104, as described above. Because the recess section 103a and the lower half cover 104 are provided, it is expected that the dust that goes through the gaps γ between the lower half 103 and the shutters 110a and 110b, and reaches the disc 102 via the opening 115 is in much smaller quantity, compared with the disc cartridge 61 (FIG. 10). However, in sliding the shutters 110a and 110b along the recess section 103, friction may occur at an edge section 115a of the opening 115, thereby producing friction powder. The friction powder may reach the disc 102 via the opening 115 and attach onto the surface of the disc 102, as in the disc cartridge 61 shown in FIG. 10, and may hinder the recording and reproduction.
Further, Japanese Publication of Unexamined Patent Application, Tokukai, No. 2001-126432 (published on May 11, 2001) discloses a disc cartridge provided with a rib in a vicinity of a head inserting inlet (through which a head is inserted), which corresponds to the openings. A shutter slides on the rib. In this publication, a contact surface between the shutter and rib is reduced so as to reduce the occurrence of the production of friction powder due to friction between the shutter and the rib; nevertheless, it is difficult to prevent the occurrence of the production of friction powder. In short, in the disc cartridge of the publication, it is inevitable that the friction powder is produced due to the sliding of the shutter along the rib, and the resultant friction powder passes through the head inserting inlet and to attach onto the surface of the disc.